Vibrational excitation through tug-of-war inelastic collisions.
Stuart J. Greaves, Eckart Wrede, Noah T. Goldberg, Jianyang Zhang, Daniel J. Miller, and Richard N. Zare.
Nature 454, 88−91 (2008). abstract full text
See also the News & Views feature by Mark Brouard:
When molecules don't rebound.
Mark Brouard, Nature 454, 43−45 (2008). full text
We employ the quasi-classical trajectory (QCT) methodology as described in our two recent papers:
New, Unexpected and Dominant Mechanisms in the Hydrogen Exchange Reaction. JCP 128 164306 (2008).
A quasi-classical trajectory study of the time-delayed forward scattering in the hydrogen exchange reaction. JCP 128 164307 (2008).
5 million trajectories for H + D2(v = 0, j = 0) collisions were propagated at a collision energy of Ecol = 1.72 eV. Inelastically scattered trajectories for the D2(v' = 3, j' = 0,2,4,6,8) product quantum states were analysed in more detail for comparison with experimental data by Zare and coworkers.
Figure 1 below [adapted from Figure 2 of Nature 454, 88−91 (2008)] shows the almost linear correlation of the impact parameter of the H atom before and the deflection angle of the H atom after the collision for the D2(v' = 3, j' = 0) product state. Trajectories with low impact parameter are backwards scattered (the H atom turns around) whereas trajectories with high impact parameter are forward scattered (the H atom carries on in the same direction). Most trajectories are scattered on the nearside, i.e. into the same hemisphere as the incoming H atom.
Click on the links below or on the numbered areas of Figure 1 to start the corresponding trajectory movies.
|b / Å||Θ / deg|
|Figure 1: Deflection angle, Θ, versus impact parameter, b, for D2(v' = 3, j' = 0) inelastically scattered products from QCT calculations of H + D2(v = 0, j =0) collisions at a collision energy of 1.72 eV.|
The trajectory animations clearly show that the D-D bond is stretched due to the attractive forces between the incoming H atom and the nearest D atom in the molecule. The vibrational excitation of the D2 results from the "tug" of the H atom as it departs and not, as is common wisdom for inelastic collisions, by compressing the D-D bond upon impact. Thus, the vibrational excitation is the result of a "tug of war" between the H atom and the D2 molecule. These inelastic collisions are frustrated reactive scattering events in which the well between H and D never deepens enough to break the D-D bond to lead to HD products.
The same behaviour, i.e., that the product motion is set up by the final pull of the departing H atom, is seen in trajectories leading to higher rotational excitation. Figure 2 shows the scattering angle – impact parameter correlation for D2(v' = 3, j' = 4) products which exhibits the same linear trend as for j' = 0, although less well defined.
|b / Å||Θ / deg|
|Figure 2: Deflection angle, Θ, versus impact parameter, b, for D2(v' = 3, j' = 4) inelastically scattered products from QCT calculations of H + D2(v = 0, j =0) collisions at a collision energy of 1.72 eV.|
|Reaction mechanisms||RDSG home||E Wrede home|
Eckart Wrede, 28 June 2008